Tanning is already unsafe. It has been linked to skin cancer. It is inadvisable to tan more than twice per week, though people do go more often than that. Also, pay attention to the amount of time you are in the tanning bed.
2 Additional Answers
The Surgeon General does not recommend tanning at all due to the risk of skin cancer. However, if you must 'fake bake', then most tanning salons will not allow you to go more than 3 to 4 times a week.
In my opinion going tanning three times a week is about the most one should go and only for about 10 minutes. Also be sure that you always put lotion on.